1. Field of the Invention
The present invention relates to a method and system for controlling power distribution in a hybrid fuel cell vehicle.
2. Background Art
A hybrid fuel cell vehicle may include three power sources for its electrical loads: a battery, the fuel cell, and a traction motor. While powering the vehicle, the traction motor is a load, but during coast-down, the traction motor becomes a generator. This regenerative power can supply current to the other loads or be used to charge the battery. Coordinating the current flow between these sources and the continually varying electrical loads presents a fundamental control problem. Imprecise control can result in reduced fuel economy, poor performance, reliability problems, and possible electrical bus instabilities.
In addition, there are other considerations when utilizing a fuel cell in a power distribution system. For example, unlike a battery, a fuel cell may not be able to instantly supply sufficient current to meet the needs of an increased electrical load. Therefore, the battery needs to xe2x80x9cfill inxe2x80x9d current temporarily, then taper off when the fuel cell""s current output increases. Without the presence of the battery to temporarily supply current, performance may degrade. In addition, the battery may also provide a repository for excess fuel cell current and regenerative current during braking and coast down.
One attempt to integrate a battery into a hybrid fuel cell vehicle is described in SAE Paper No. 2002-01-0096, titled xe2x80x9cDevelopment of Fuel-Cell Hybrid Vehiclexe2x80x9d (xe2x80x9cthe SAE Paperxe2x80x9d). The SAE Paper describes the use of a battery connected in parallel with fuel cells via a DC/DC converter. The battery is configured to provide a power assist when fuel cell response is delayed, or when the vehicle is driven under high load conditions. The traction motor is located between the fuel cell and the converter; whereas, the fuel cell auxiliary systems are located between the battery and the converter. To determine the fuel cell operational point, power-current (P-I) and current-voltage (I-V) maps are used. A power requirement is input, and using the P-I and I-V maps, a voltage command is determined.
One limitation of the hybrid vehicle described in the SAE Paper is its use of operating modes which do not utilize the fuel cell, but rather, rely solely on the battery to supply all of the power. In such operating modes, all of the vehicle electrical loads are carried by the battery. This may require the use of an undesirably large battery, or place limits on the loads the system is able to handle. In addition, the SAE Paper does not describe a system or method for controlling the rate of change of current flow to or from the battery, nor does it describe how to determine a target rate.
Accordingly, there exists a need for a method and system that provide for controlling power distribution in a hybrid fuel cell vehicle such that a fuel cell works in conjunction with a second power source, such as a battery, ultra-capacitor, or other equivalent electrical storage device, to provide power to vehicle electrical loads, and a system equilibrium is sought, wherein the fuel cell carries all of the vehicle electrical loads, and the current flow of the second power source is adjusted at a target rate until a predetermined constant is reached.
Therefore, a power distribution control system for a vehicle having a fuel cell and a second power source connected to an electrical bus is provided. The control system includes a voltage regulator configured to control voltage on the bus. A first controller controls the voltage regulator. The first controller is configured to determine a voltage command and send it to the voltage regulator. The voltage command is related to a target current flow for the second power source, and also related to a target rate for reaching the target current flow. A second controller is provided for controlling the fuel cell. The second controller is configured to provide a fuel cell current request to the fuel cell, and further configured to provide an input to the first controller. The input is related to available fuel cell current.
Some embodiments of the invention also include a power distribution control system having electrical loads connected directly to the fuel cell, which provides a low cost, efficient architecture. Since main power current can go directly from the fuel cell to the loads without passing through another device, the battery and voltage regulator size can be minimal. This may result in an overall cost savings.
In addition, some embodiments of the invention provide a fast, inner control loop to quickly respond to initial transients, and a slower, outer control loop to help ensure smooth transitions as the vehicle electrical loads change. Moreover, embodiments of the invention may include a gain scheduler capable of providing individual response tunings, and an adaptive polarization curve, both of which may increase the response and stability of the power distribution control system.
The invention also provides a method of controlling the power distribution in a vehicle having a fuel cell and a second power source connected to an electrical bus. The method includes adjusting current flow of the second power source a first time at least partly based on a vehicle electrical load change. Available fuel cell current is adjusted partly based on the vehicle electrical load change. The current flow of the second power source is continuously adjusted until an equilibrium point is reached. The continuous adjustment is at least partly based on an actual rate of change of the available fuel cell current.
The invention further provides a vehicle having a fuel cell and a second power source connected to an electrical bus, and a power distribution system for controlling the distribution of power in the vehicle. The power distribution system includes a voltage regulator configured to control voltage on the bus. A first controller is provided for controlling the voltage regulator. The first controller is configured to determine a voltage command and send it to the voltage regulator. The voltage command is related to a target current flow for the second power source, and also related to a target rate for reaching the target current flow. A second controller is provided for controlling the fuel cell. The second controller is configured to provide a fuel cell current request to the fuel cell, and further configured to provide an input to the first controller. The input is related to available fuel cell current.
The invention also provides a computer programmed and configured to execute control algorithms for controlling the power distribution system in a vehicle. The vehicle has a fuel cell and a second power source. The computer includes an algorithm for adjusting current flow of the second power source a first time in response to a vehicle electrical load change. The computer includes an algorithm for adjusting available fuel cell current at least partly based on the vehicle electrical load change. The computer also includes an algorithm for continuously adjusting the current flow of the second power source until an equilibrium point is reached. The continuous adjustment of the current flow is at least partly based on an actual rate of change of the available fuel cell current.
The above objectives, features, and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.